Software systems that use relational databases to support persistence of data typically have to deal with a number of complex SQL queries, and need a lot of logic to implement fetching of different query results and create the corresponding data structures.
In this context, a very common problem is that the amount of code that executes fetching of data from database queries increases with the number of queries, and the more the application uses complex SQL queries, the harder it is to reuse this code. Even if trying to use base functions to fetch subsets of the query results independently, there is still the need to write for each query some “glue” code that assembles all the results returned from base functions into the final expected structure.
The Java Platform, Enterprise Edition or Java EE (formerly known as Java 2 Platform, Enterprise Edition or J2EE up to version 1.4), is a programming platform for developing and running distributed multi-tier architecture Java applications, based largely on modular software components running on an application server (Java and all Java-based trademarks and logos are trademarks of Sun Microsystems, Inc. in the United States, other countries, or both.)
Within the context of J2EE applications, Enterprise JavaBeans (EJBs) are one of the several specifications defined by the J2EE standard. An EJB is a server-side component that encapsulates the business logic of an application, which can run in a special environment called an EJB container. The EJB container manages every aspect of an enterprise bean at runtime including remote access to the bean, security, persistence, transactions, concurrency, and access to and pooling of resources.
With container-managed persistence, the programmer defines how to map the representation on the Java platform to the database model, and the container generates the required code to implement the mapping. While defining this mapping, the programmer has the ability to specify multiple tables and relationships and their mappings to the EJBs, so that the container can generate queries involving multiple tables. However, the EJB container can automatically generate only simple queries involving a small number of tables, and has several limitations in the ability to create and manage complex queries involving subqueries, dynamic clauses, inheritance relationships, etc.